Conventional pick arm arrangements generally have little difficulty in picking media having low stiffness and low media to media coefficients of friction. However, when the media is stiff and/or the media to media coefficient of friction is high, the resistant force increases significantly. This problem magnifies when the pick arm is an auto compensation pick arm. U.S. Pat. No. 5,527,026 describes such a pick arm.
The resistant force in a media picking process consists of two components. The first component is the frictional force between the media being picked and an adjacent media on the media stack, which is directly proportional to the coefficient of friction (COF) between the two surfaces and the normal force applied to the media stack by the media roller. The normal force component is proportional to the applied torque at the point of time.
The second component is equal in magnitude, but opposite in direction, to the force applied by the media roller onto the media less the frictional force between media (which is the first type of resistant force mentioned above). The maximum value of this second component is the force required to buckle/curl the media up the separation wall, which is fixed for a certain geometrical set up and only varies with the type of media used. It therefore will be appreciated that it becomes much more difficult to pick the abovementioned media and drive them up the separation wall of the apparatus. Accordingly, a higher drive torque needs to be applied to the pick arm.
For a set up where the roller to wall distance is short, the resistant force is high due to the higher buckling force required to buckle the media. This problem becomes more significant for an auto compensation pick arm. As the drive torque on the auto compensation pick arm is increased, the geometry of the arrangement is such that the reaction force of the media roller on the media is also increased. With some auto compensation pick arm arrangements when faced with relatively high resistant force, the normal force component will increase significantly as compared to the drive component. This means that excessively high torque needs to be applied in order to generate a slight increase in drive force. This limits the capability of such arrangements to pick up media with high resistant force, as excessive high torque can cause part damage.
There are two common ways to resolve this problem. The first is to reduce the angle of the separation wall of the apparatus so as to allow the media to more readily ascend the separation wall. The second is to shift the media roller further away from the separation wall so that the resistant force is reduced. Both methods reduce the resistant force required to buckle/curl the media up the separation wall.
Unfortunately, when either of the above solutions is adopted a much higher frequency of multiple media pick ups occurs with low stiffness media. Thus it becomes necessary to either adjust the angle of the separation wall or to shift the media roller relative thereto depending on the nature of the media being used in the apparatus. To achieve this automatically it is necessary to have the ability to accurately and reliably detect the media type in the media tray (most likely by means of some sensors) and include a motor to change the angle of the separation wall or to move the media roller. The inclusion of such a motor results in increased apparatus costs and increased design complexity.
Another approach to resolve this problem is to spring bias the system and alter the geometry, by further stressing the spring, when certain forces in the system exceed certain predetermined values. One example of such a system uses a pick arm mounted on a movable platform which is fitted on two slider tracks. When the drive force exceeds a certain value, the platform is caused to slide to a new position where it is further away from the separation wall. By doing this, the distance between the pick roller and the separation wall is increased and thereby the buckling force is reduced. After the media is moved, the platform is returned to its original position under the action of the spring bias.
U.S. Pat. No. 6,322,065 describes a pick arm formed from an inner section and an outer section which are hinged together at a spring biased hinge axis. The end of the inner section is mounted at a pivot point. A motor mounted on the pick arm drives a pick roller mounted at the end of the outer section. During rotation of the pick roller, the media applies a resisting force to the pick roller which induces a moment at the pivot point of the inner section. When the moment exceeds the spring force, the inner and outer sections hinge at the hinge axis and rotate. The inner section rotates in a clockwise direction and the other section rotates in an anticlockwise direction. As a result, the pick roller is moved away from the separation wall.
One advantage of hinging the pick arm is that picking becomes more effective as the angle of the lower section α increases. This is because as the pick arm angle α increases, the normal force increases. Unfortunately, by increasing the normal force the device of U.S. Pat. No. 6,322,065 reduces its ability to pick “sticky media” (i.e. media with a high sheet to sheet coefficient of friction). This is a significant disadvantage.
The present invention seeks to address at least some of the problems identified above.